This invention is directed to a method for preheating a molten salt electrolysis cell so as to avoid early failure of protruding electrode elements of such a cell due to the effects of thermal gradients encountered.
The invention may be employed in connection with electrolytic reduction cells used in the production of aluminum. Such cells may operate by electrolytically reducing alumina to aluminum.
A conventional cell of this type usually includes a thermally insulated steel box lined with blocks of carbon cemented together with a carbon paste. Alternatively, the cell lining may be of the monolithic variety, which consists of a rammed mixture of finely ground carbon and pitch. The carbon floor of the cell transmits electrical current from a molten pad of aluminum which serves as the cathode surface; the anode surface is provided by carbon blocks suspended from above.
In operation, the conventional cell contains an electrolytic, molten cryolite-based bath in which alumina is dissolved. A crust of frozen electrolyte and alumina forms on top of the bath and around the anode blocks. As electrical current passes through the bath between the anode and cathode surfaces, alumina is reduced to aluminum, which is deposited in the pad of molten metal.
Electrolytic reduction cells must be heated from room temperature to approximately the desired operating temperature before the production of metal can be initiated. For cells used in the reduction of aluminum from alumina, the desired operating temperature is usually in the range of 900.degree.-1000.degree. C. The heating operation may be employed to bake monolithic linings or to bake the carbon paste seam cement where prebaked carbon blocks are employed. In addition, heating prior to operation is necessary to minimize thermal shock to the lining upon introduction of the electrolyte and molten metal to the cell.
In the case of conventional reduction cells having a monolithic lining, the initial heating operation must include a preliminary baking step. It has been known to bake these linings by building a wood fire in the cell. This method of baking has the advantage of allowing baking of the lining to proceed while driving off and even burning the volatile components of the pitch in the lining mixture. Other baking methods have involved an attempt at baking the monolithic linings by means of "resistance heating" methods, such as are commonly employed to heat reduction cells to the desired operating temperature.
One such resistance heating method, useful in the heating of a conventional reduction cell to its desired operating temperature, is set forth in Tilson's U.S. Pat. No. 1,572,253. This method involves lowering the carbon anodes into contact with the carbon floor, and then passing a flow of current from the anode through the cathode and out of the cell through the cathode connector terminals. According to this method, the flow of current through the "high resistance encountered at the points of contact" between the anodes and the carbon floor causes "rapid and great evolution of heat at such points."
It is also known to heat a conventional cell by passing electrical current through a layer of carbon particles interposed between the anode blocks and the carbon floor of the cell. Such a heating method is discussed in British Pat. No. 1,046,705, of the British Aluminium Company, Limited. This method calls for a one to two inch layer of carbon particles to be placed on the carbon floor. The anode blocks are then lowered to contact this layer, and current is passed through the particles to the carbon floor.
Although these known methods of heating molten salt electrolysis cells may provide satisfactory results in preparing conventional cells for operation, they are not suitable for low temperature heating of the improved cell contemplated by this invention.
Such a cell is described in U.S. Pat. No. 4,071,420, of Foster et al., issued on Jan. 31, 1978. This cell accommodates the electrolysis, between anodic and cathodic surfaces, of a compound of a metal such as aluminum dissolved in a molten solvent. In the case of the electrolysis of alumina, the molten solvent may be cryolite-based. The electrolysis is performed at a temperature such that the metal is formed in the molten state, and the metal thus formed collects in a molten metal pad. In one embodiment, the cathode of this cell is provided in the form of an array of elements that protrudes out of the metal pad into the solvent toward the anode. There is thus established a series of locations at which the distance between the anode and cathode surfaces is preferably less than or equal to 11/4 inches, thereby providing for more advantageous operation of the cell.
The cathode elements of the cell of Foster et al. are preferably made from sintered composites of refractory hard metals, such as, for example, titanium diboride (TiB.sub.2). These elements survive the conditions of cell operation very well once the desired operating temperature is reached, but they are somewhat sensitive to damage caused by thermal gradients encountered at relatively low temperatures. The temperature above which the elements are less sensitive to thermal gradients is not easily calculated, because it varies, depending upon the physical properties of the material of which the elements are made, the size and shape of the elements, the placement of elements in the cell, the shape of the cell itself, and the rate at which the cell is heated.
The known methods of heating an electrolysis cell, involving the passage of current from the anode directly to the carbon floor or through an interposed layer of carbon particles to the floor, are unsuitable for heating the cell of Foster et al. until the cell has been raised to an elevated temperature. When employed to heat this cell at relatively low temperatures, these resistance heating methods produce localized "hot spots" on the surfaces of the cathode elements nearest the anode unless carried out at very low heating rates over a long period of time. This uneven heating of the elements may lead to thermal shock breakage or spalling.
This damage may occur in the following manner. At relatively low temperatures, during heating of the elements by resistance methods at commercially practical heating rates, the surfaces nearest the anode are heated more rapidly than other portions of the elements. This may be due to the fact that the small surface area of the elements near the anode relative to the area of the cell floor leads to the development of higher current densities near those areas. In any event, during resistance heating of the elements at low temperatures, a large nonlinear temperature gradient develops between the surfaces of the elements nearest the anode and those surfaces farthest away. This gradient causes differential expansion of the elements, giving rise to differential stresses. When such stresses exceed the strength of the elements, cracking may occur at varying distances from the anode. Severe cracks may cause portions of the elements to break away. Obviously, the performance of a severely cracked or spalled element is substantially poorer than the performance of a similar, undamaged element. In addition, an element damaged by spalling tends to wear faster than an undamaged element. In fact, such a damaged element may completely disintegrate during cell operation.
Because of the problem of spalling that occurs when protruding refractory hard metal electrode elements are heated by conventional resistance methods, the British Aluminium Company has developed a modified resistance heating method for use in connection with cells employing such elements. This modified method, which is described in the aforementioned British Pat. No. 1,046,705, involves passing a flow of current from a carbon electrode which is intended to function as an anode during normal cell operation, through a carbonaceous resistance heating material, to another electrode which is intended likewise to function as an anode during normal cell operation.